(1) Field of the Invention
The present invention relates generally to adaptive-array-beamforming processors and, more specifically, to adaptive-array-beamforming processors having a data-based technique that, relative to prior art, improves the description of the amplitude and phase of acoustic signals, as they propagate across an array of sensor elements, for each array incident angle of interest. The appropriate data-based determination of the acoustic signal is then used to constrain adaptive noise suppression algorithms, such that acoustic signals in the desired incident (or steering) directions are canceled less than that of prior art. In direction where there are no acoustic signals and the noise is largely uncorrelated, the use of data-based signal definitions also reduces noise more than that of prior art. The use of the data-based technique, within an adaptive-array-beamforming processor, results in an improvement in the ability to detect acoustic signals imbedded in noise.
(2) Description of the Prior Art
An adaptive-array-beamforming prior art processor 10 is used to detect acoustic signals imbedded in noise and is generally illustrated in FIG. 1. An array 12 used with the processor 10 consists of several sensor elements that convert acoustic signal pressure waves to voltages and are represented as time series and, more particularly, as time series voltages. For frequency domain algorithms, commonly used in a processor 10, sensor element time series voltages are transformed to frequency bin voltages via well-known analyses, such as Fast Fourier Transformation (FFT) analysis.
The adaptive beamforming, that is, the processor's 10 response, which adapts itself to cancel noise for the particular acoustic signals being received, is then implemented on each frequency bin processed. In operation, the array 12 is steered; that is, the array 12 is constrained by the adaptive-array-beamforming processor from canceling acoustic signals in a steered or desired incident direction. The terms “steered direction” and “desired incident direction” are used herein in an interchangeable manner. The steered direction is defined by a vector of complex numbers, one complex number for each sensor element. For the frequency domain algorithm, a steering vector is required for each frequency bin processed. The constraint for steering the array requires that the sum of the product of the steering vectors and the computed adaptive weights for each sensor element be unity for each frequency bin. The adaptive weight computation, which gives less or near zero significance to all directions other than the steering direction, is well known in the art. The term “actual signal for detection” means a real-world signal encountered by a signal system for the detection thereof. The adaptive-array-beamforming-processor to which the present invention relates is a stage in the signal system prior to the apparatus performing the detection. An example of an “actual signal for detection” in the signal system context of a sonar system used on a naval submarine would be the acoustic signal generated by a torpedo in the course of the torpedo attacking the submarine. The acoustic signal of the torpedo may be imbedded in noise caused by the underwater environment and/or other causes. Such an “actual signal for detection” stands in contradistinction to the artificially generated signals projected toward the array for calibration purposes in the disclosed embodiment of the earlier cited related patent application Ser. No. 09/922,308, wherein the artifical signals are produced by a signal generator (20, FIG. 2 therein) in response to a signal projector controller (22, FIG. 2, therein).
The prior art suffers drawbacks in developing constraints for adaptive-array-beamforming-processors because the prior art assumes free-field propagation of acoustic signals to each sensor element within the array 12. The result is a free-field based steering vector arrangement 14 that is used to disadvantageously constrain the adaptive-array-beamforming-processor by canceling acoustic signals representative of a desired incident direction. For example, with nearby array mounting structure scattering acoustic signals (as well as amplitude and phase variations inherent in sensor element manufacture), the signal amplitude and phase at each sensor element can deviate from free field propagation. This deviation makes the uses of the signal amplitude and phase of each sensor as an error contribution in constraining the adaptive-array-beamforming-processor causing the cancellation of desired acoustic signals. It is desired that means be provided for developing constraints for adaptive-array-beamforming processors that do not cancel the handling of desired acoustic signals as much as that of prior art so that increased signal-to-noise at the output of the beamforming processors results.